1. Technical Field
The present disclosure relates to an electrosurgical system and method. More particularly, the present disclosure relates to an apparatus and method for tissue ablation having increased electrode temperature control.
2. Background of Related Art
Therapeutic lesions in living bodies have been accomplished for many decades using radio-frequency (RF) and other forms of energy. The procedures have been particularly useful in the field of neurosurgery, typically where RF ablation electrodes (usually of elongated cylindrical geometry) are inserted into a living body. A typical form of such ablation electrodes incorporates an insulated sheath from which an exposed (uninsulated) tip extends.
Generally, the ablation electrode is coupled between a grounded RF power source (outside the body) and a reference ground or indifferent electrode for contacting a large surface of the body. When an RF voltage is provided between the reference electrode and the inserted ablation electrode, RF current flows from the ablation electrode through the body. Typically, the current density is very high near the tip of the ablation electrode, which heats and destroys the adjacent tissue.
Ablation electrode techniques, including the theory behind the techniques and many applications of the techniques are described in various papers, specifically see, (1) Cosman et al, “Theoretical Aspects of Radiofrequency Lesions in the Dorsal Root Entry Zone” Neurosurg 15:945-950, 1984 and (2) Cosman E. R. and Cosman B. J.: “Methods of Making Nervous System Lesions, in Wilkins R H, Rengachary S S (EDS): Neurosurgery, New York, McGraw-Hill, Vol. III, pp. 2490-2498, 1984.
In the past, RF ablation electrodes have incorporated temperature sensors, for example, in the form of a thermistor or thermocouple. In that regard, see U.S. Pat. No. 4,411,266 (1983, Eric R. Cosman). Typically, the sensor is connected to a monitoring apparatus for indicating temperature to assist in accomplishing a desired lesion. As generally known, for a given tip geometry and tip temperature, lesions of a prescribed size can be made quite consistently. In that regard also, see U.S. Pat. No. 4,411,266, (1983, Eric R. Cosman).
Over the years, a wide variety of RF electrode shapes and configurations have been used, for example, several current forms are available from ValleyLab Inc., Boulder, Colo. Such electrodes have been used to accomplish lesions in a wide variety of targets within the body, including the brain, the spinal column and the heart.
However, a limitation of prior electrode ablation systems relates to the temperature of the tip. Specifically, prior ablation electrodes of a given tip geometry should not effectively exceed a temperature of 100.degree. C. At that temperature, the surrounding tissue will boil and char. Also, uncontrolled disruption, such as hemorrhage and explosive gas formation, may cause extremely hazardous and clinically dangerous effects on the patient. Consequently, the lesion size for a given electrode geometry generally has been considered to be somewhat limited by the fact that the tissue near the tip must not exceed 100.degree. C.
Essentially, during RF ablation, the electrode temperature is highest near the tip, because the current density is the highest at that location. Accordingly, temperature falls off as a function of distance from the electrode tip, and except for possible abnormalities in tissue conductivity, in a somewhat predictable and even calculable pattern. As an attendant consequence, the size of RF lesions for a given electrode geometry have been somewhat limited.
One proposed solution to the limitation of lesion's size has been to employ “off-axis” electrodes, for example the so called Zervas Hypophysectomy Electrode or the Gildenberg Side-Outlet electrode, as manufactured by Integra Radionics, Inc, Burlington, Mass. However, such systems in requiring multiple tissue punctures, increase the risk of hemorrhage, severely prolong the time of surgery and increase the level of delicacy. An umbrella of off-axis lesions may not produce a desired homogenous or uniform lesion.